Error fetching OAuth2 access token, invalid_request, with php google client api

I know I'm not the only one having this problem, but untile now I wasn't able to find a solution. Please help!

I'm experimenting with the php google client api to use it in my site. But I cannot authorize my request using $client->authenticate(); my page gets stuck with this error:

PHP Fatal error: Uncaught exception 'Google_AuthException' with message 'Error fetching OAuth2 access token, message: 'invalid_request'' in /Users/****/Sites/googleApiClient/google-api-php-client/src/auth/Google_OAuth2.php:113 Stack trace:

#0 /Users/****/Sites/googleApiClient/google-api-php-client/src/Google_Client.php(131): Google_OAuth2->authenticate(Array, NULL)

#1 /Users/****/Sites/googleApiClient/index.php(90): Google_Client->authenticate()

#2 /Users/****/Sites/googleApiClient/index.php(124): init()

#3 {main}

thrown in /Users/Angelo/Sites/googleApiClient/google-api-php-client/src/auth/Google_OAuth2.php on line 113

Here is my code (sism_debug() is something I use to var_export to the console):

<?php
function sism_debug($var){

    $result = var_export( $var, true );

    $trace = debug_backtrace();
    $level = 1;
    @$file = $trace[$level]['file'];
    @$line = $trace[$level]['line'];
    @$object = $trace[$level]['object'];
    if (is_object($object)) { $object = get_class($object); }

    error_log("Line $line ".($object?"of object $object":"")."(in $file):\n$result");
}

function retrieveAllFiles($client) {

    require_once "google-api-php-client/src/contrib/Google_DriveService.php";

    $service = Google_DriveService($client);

    $result = array();
    $pageToken = NULL;

    do {
        try {
          $parameters = array();
          if ($pageToken) {
            $parameters['pageToken'] = $pageToken;
          }
          $files = $service->files->listFiles($parameters);

          $result = array_merge($result, $files->getItems());
          $pageToken = $files->getNextPageToken();
        } catch (Exception $e) {
          error_log("An error occurred: " . $e->getMessage());
          $pageToken = NULL;
        }
    } while ($pageToken);
    return $result;
}

session_start();

global $access_token;

error_log("What's in the SESSION global?");
sism_debug(@$_SESSION['access_token']);

$access_token = @$_SESSION['access_token'] || false;

function init(){
    // Richiama la libreria di Google
    require_once 'google-api-php-client/src/Google_Client.php';

    // Crea un oggetto per immagazzinare i dati inseriti nel form. Qui andrebbero usate le funzioni di sanitizzazione degli input di WP
    $app_data = array();

    @$app_data['client_ID'] = $_POST['client_ID'];
    @$app_data['client_secret'] = $_POST['client_secret'];
    @$app_data['scopes'] = explode("\n",trim($_POST['scopes']));
    @$app_data['redirect_url'] = 'http://' . $_SERVER['HTTP_HOST'] . $_SERVER['PHP_SELF'];

    // Confronta i nuovi dati con quelli salvati nel database
    if($_SESSION['app_data'] !== $app_data){
        $_SESSION['app_data'] = $app_data; //Salva i dati nel database
    }

    $client = new Google_Client();
    $client->setClientId($app_data['client_ID']);
    $client->setClientSecret($app_data['client_secret']);
    $client->setRedirectUri($app_data['redirect_url']);
    $client->setScopes($app_data['scopes']);
    $client->setAccessType('offline');

    //Numero random di sicurezza. In WP si usa wp_nonce()
    if(!isset($_SESSION['security']) OR $_SESSION['security'] == ""){
        $_SESSION['security'] = md5(rand());
    }

    $state = $_SESSION['security'];

    $client->setState($state);
    $client->setApplicationName('Google Api Client');

    error_log('Client Set!');

    if (isset($_GET['code']) AND $_GET['state'] == $state AND (!isset($access_token) OR !$access_token)){
        error_log('Code arrived!');
        sism_debug($_GET);

        error_log('Now authenticating...');
        $result = $client->authenticate(); // Get stucked here!
        sism_debug($result);
        $access_token = $_SESSION['access_token'] = $client->getAccessToken();
        sism_debug($access_token);
    }

    if (!isset($access_token) OR !$access_token){
        error_log("Creating authUrl...");
        $authUrl = $client->createAuthUrl();
        sism_debug($authUrl);
        header('Location: ' . $authUrl);
        exit();
    }
    else{
        // Se richiesto il logout cancellare i dati registrati
        if (isset($_POST['logout']) AND $_POST['logout'] == "true") {
            error_log('Logging out!');
            $client->revokeToken();
            session_destroy();
        }
        else{
            error_log('Setting up access token...');
            $client->setAccessToken($access_token);
        }
    }

    if ($client->getAccessToken()) {
        error_log('Fetching Results');
        sism_debug(retrieveAllFiles($client));
    }
}

//Controlla che la pagina sia stata chiamata tramite un Submit del form o redirect di google
if( (@isset($_POST['manual_submit']) AND @$_POST['manual_submit']) OR (@isset($_GET['code']) AND @$_GET['code'])){
    error_log('Manual submit or google redirect! Initialiazing...');
    init();
}
elseif (@isset($_POST['error'])){
    error_log('Something got wrong...');
    sism_debug($_POST['error']);
}
error_log('Rendering page...');
?>


<!DOCTYPE html>

<html>
<head>
    <meta charset="utf-8">
    <title>GoogleApiClient</title>
    <style type="text/css">
        .hidden {
            display: none;
        }
    </style>
</head>

<body>
<h1>Google Api Client</h1>
<p>L'applicazione proverà ad accedere ai tuoi dati</p>
<a href="http://<?php echo $_SERVER['HTTP_HOST'] . $_SERVER['PHP_SELF'] ?>">link alla pagina</a>
<form action="index.php" method="POST">
    <p>
        <label>Client ID</label><br>
        <input type="text" name="client_ID"/>
    </p>
    <p>
        <label>Client Secret</label><br>
        <input type="text" name="client_secret"/>
    </p>
    <p>
        <label>Scopes</label><br>
        <textarea name="scopes"></textarea>
    </p>
    <input type="hidden" name="manual_submit" value="true"/>
    <?php if(!isset($access_token) OR $access_token == ""){?>
    <input type="submit" value="Autorizza"/>
    <input type="hidden" name="auth_action" value="authorize"/>
    <?php }
    else{ ?>
    <input type="submit" value="Revoca"/>
    <input type="hidden" name="auth_action" value="logout"/>
    <?php } ?>
</form>

<script src="http://code.jquery.com/jquery-2.0.0.min.js"></script>
<script type="application/x-javascript">
    $(document).ready(function() {
        $("input:text, textarea").each(function(){
            $(this).val(localStorage[$(this).attr('name')]);
        })

        $("form").submit(function(){
            console.log("form submit!");
            $("input:text, textarea").each(function(){
                localStorage[$(this).attr('name')] = $(this).val();
            })
        });
    });
</script>
</body>
</html>

As you can see, this script allows you to set your credential up in the front end.

Here is the request when asking for the code:

client_ID:****.apps.googleusercontent.com
client_secret:****
scopes:https%3A%2F%2Fwww.googleapis.com%2Fauth%2Fdrive
manual_submit:true
auth_action:authorize

As I said, then it gets stuck on authentication, but I don't know which form the request has, since is managed by the google client api.

Any idea??

Thanks!